lib/b64dec: Use grub_size_t instead of size_t for _gpgrt_b64dec_proc() function defin...

lib/b64dec: Use grub_size_t instead of size_t for _gpgrt_b64dec_proc() function definition

On some targets, size_t and grub_size_t may not be the same type
(unsigned long / unsigned int). This breaks the compilation because the
definition of _gpgrt_b64dec_proc() differs from gpgrt_b64dec_proc()
declaration. Fix it by using grub_size_t in the _gpgrt_b64dec_proc()
definition.

Signed-off-by: Anaëlle Cazuc <acazuc@acazuc.fr>
Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>